As telecommunication networks continue to expand their need for bandwidth, it is becoming increasingly necessary to introduce new technologies to keep up with growing demands. These technologies should not only facilitate the need for bandwidth but also be easily incorporated into today's network infrastructure. At the same time, they should be flexible and versatile enough to fit the requirements of the future. While current telecommunication systems comprise a combination of electronic and optical data-transmission, there is pressure to move towards all-optical networks due to the increased bandwidth provided by high bit-rates and parallel transmission through wavelength division multiplexing.
Currently, optical networks use light for much of the transmission of data between nodes in an optical circuit. Optical cross-connects function as switches in these nodes by routing signals arriving at one input-port to one of a variety of output-ports. Most current optical cross-connect systems comprise high-speed electronic cores, which are complex, cumbersome, and expensive. These switches typically require a light signal to be translated into an electronic signal, which is switched or routed to an output-port before being reconverted to a light signal. The complexity, size, and expense of such optical-to-electronic-to-optical (OEO) components become even more problematic with higher bit-rates and port counts, even as the cost of electronic components decreases, due to cross-talk and RF transport issues.
OEO devices are typically the rate-limiting component in an optical network. As such, many options are being considered to reduce the need for both OEO conversions, as well as electronic-signal processing in optical network components. This has lead to emphasis being placed on the development of “all-optical” switching technology, in which optical signals passing through a switch are diverted to the appropriate destination without being converted to electronic signals.
For most current applications, electronically controlled optical cross-connects with optical-cores can be used as an all-optical switch. In these devices, light routing does not require OEO conversion, but operation of the switch is electronically controlled. The various all-optical switching technologies that currently support such systems include electromechanical switches (e.g., MEMS or bulk optics), thermo-optic switches (e.g., phase shift, capillary, or “bubble”), and electro-optic switches (e.g., LiNbO3 or liquid crystal). In addition, a variety of nonlinear optical switches (e.g., semiconductor optical amplifiers) use a light beam, rather than electronics, to operate the switch.
Many all-optical switching technologies are relatively slow and are therefore generally limited to static configuration control. For example, applications such as basic fiber/wavelength routing, provisioning, and restoration typically require switching speeds around 1 ms. These relatively slow all-optical switches, however, are generally inadequate for fast switching applications such as dynamic packet switching (˜1 ns), optical modulation (˜100 ps), header reading in packet switched networks (<25 ps), and all-optical data-processing (<1 ps).
Currently, devices based on electric field-induced optical changes, such as the electro-optic effect (a χ(2) effect) and electro-absorption (a χ(3) effect) are utilized for optical modulation and switching. However, these devices are rapidly approaching their speed limits, as they rely on fast electronic signals in order to perform optical processing or modulation, and these electronic signals suffer increasingly greater losses due to the fundamental limitations of high-speed electrical propagation. Devices based on nonlinear optical phenomena, such as cross-gain modulation (XGM) in semiconductor optical amplifiers, χ(2) based phenomena (e.g., difference-frequency mixing (DFM)), and χ(3) (or Kerr) based phenomena (e.g., cross-phase modulation (XPM) and four-wave mixing (FWM)), have the potential to switch at rates required for packet-switching, optical data processing, and other future high-speed switching applications. Devices based on such phenomena have the potential (depending on the mechanism) for switching speeds approaching (and even exceeding) ten terabits per second (10 Tbit/s), or 10 trillion bits per second. Of these nonlinear optical phenomena, χ(3) based phenomena have the most flexibility but currently suffer from a lack of practical materials with both high nonlinearity and relatively low loss.
Research involving the development of χ(3) based all-optical devices has been extensively pursued since the mid-1980s and has primarily focused on silica fiber-based devices. This is due to the relatively large figure-of-merit (FOM) for nonlinear optical switching for silica. There are many practical definitions of a FOM that take into account the many parameters that can be important and relevant to all-optical switching. One example of such a FOM is defined as             Δ      ⁢                           ⁢      n              α      ·      τ        ,where Δn is the induced refractive index change, α is the linear and nonlinear absorption coefficient, and τ is the response time of the material. For this FOM, which is particularly relevant for resonant optical nonlinearities where light absorption is used, the larger the FOM, the better will be the performance of the all-optical switching. A definition of a FOM useful for nonresonant optical nonlinearities, where ideally no or little light absorption occurs, is 2γ/βλ, where γ is the nonlinear index of refraction, β is the two-photon absorption coefficient, and λ is the wavelength of operation. In this case, useful all-optical switching typically occurs when FOM>1. Due to the low linear and nonlinear losses of light at telecommunication wavelengths in silica, the FOM for silica is adequate even though Δn and γ (which are related to Re[χ(3)1111]) are small.
Many all-optical switching devices have been demonstrated using silica fiber (e.g., nonlinear directional couplers, nonlinear optical loop mirrors, and solution-based switches). Due to the small γ of silica, however, impractical fiber lengths (˜1000 km) are required for these devices to operate at typical telecommunication powers (˜10 mW). As a result, there is a great deal of interest in developing materials with both a large FOM and a large γ to reduce overall device sizes and latency. For certain applications, device sizes ˜1 mm or less are desirable for integration of multiple devices and to provide insensitivity to temperature fluctuations and manufacturing fluctuations (e.g., tight tolerance over long distances). In addition, low latency is needed as the data rates increase.
In addition to large nonlinearities with large FOMs, it is desirable that commercial optical switching components are low cost and compatible with high-throughput automated fabrication. Historically, semiconductor processing, used to make microprocessor chips, has been one of the most cost-effective and automated processes for miniaturization. While this technology is extremely advanced in the field of microelectronics, it is still in its infancy with respect to optics. For instance, for χ(2) based devices, crystalline LiNiO3 cannot be arbitrarily inserted within a waveguide created by these techniques. In addition, polymeric nonlinear materials, which are more easily processed, typically have values for χ(3) that are too low for efficient switching.
Presently, there are a variety of approaches being pursued to reduce the size of χ(3) based all-optical switches. Approaches being considered include using semiconductor optical amplifiers (SOAs), manufacturing photonic bandgap structures with nonlinear materials, enhancing nonresonant optical nonlinearities using local field effects, and developing new crystalline materials and polymeric materials with high optical nonlinearities.
While proof-of-concept for all-optical switches based on SOAs has been shown, problems with amplified spontaneous emission buildup currently make cascading many of these switches problematic. In addition, the materials used for SOAs (typically InP) are expensive and create inherent difficulties with coupling to standard silica fibers and waveguides. Photonic bandgap materials are another promising approach, but manufacturing using the previously proposed materials is still beyond current practical capabilities. While enhancing nonlinearities using local field effects is an interesting approach, enhancement factors of only ˜10× have been achieved to date. Finally, new nonlinear crystalline materials have been developed (e.g. periodically poled LiNbO3 and p-toluene sulphonate (PTS)) but are typically expensive and difficult to process, making incorporation into waveguide devices problematic. Nonlinear polymers, with more appealing mechanical properties, have also been developed, but problems such as kinks in the polymer chains can limit the maximum nonlinearity to a value still unsuitable for practical all-optical applications. In cases where highly nonlinear polymers have been produced (e.g., polyacetylene), many of the appealing mechanical properties are lost, creating problems similar to those found in crystalline materials.
In addition to high nonlinearity and processability, nonlinear materials desirably should also be low-loss in the wavelength range-of-interest (e.g., from absorption or scattering). These materials desirably should also have a linear index of refraction that is compatible with the specific architecture of the device in which they are to be used (e.g., a nonlinear waveguide core should have an index of refraction higher than the cladding surrounding it). As such, it has been extremely difficult to find a practical material that simultaneously satisfies various requirements for a commercial χ(3) based nonlinear device.
An ideal χ(3) based nonlinear optical material should have a number of characteristics, which can include the following:    1. Large Re[χ(3)ijkl] in the wavelength range-of-interest (Re[χ(3)1111] is directly related to Δn and γ).    2. Low optical losses from single- and multi-photon absorption and/or resonant and nonresonant scattering in the wavelength range-of-interest. Ideally, the photon energies corresponding to the wavelength range-of-interest are such that the two-photon absorption threshold is not met (i.e., the sum of the two photon energies are lower than the resonance energy), so that two-photon absorption and higher multi-photon absorptions are negligible.    3. A multi-photon transition near the wavelength range-of-interest such that resonant and near resonant enhancement of χ(3) occurs (but ideally no or little multi-absorption occurs).    4. A precisely selected linear index of refraction compatible with the desired application (e.g., waveguides) and intended device architecture.    5. Physical and chemical compatibility with the specific device architecture and materials with which the material will be used.    6. The ability to be processed for incorporation into optical devices.    7. Low cost of manufacturing and incorporating the material.
While many materials may have one or more of these desirable characteristics, at present, no single material comprises a sufficient number of these characteristics required for an optimal χ(3) based optical switch. In fact, besides SOAs, no commercial devices are currently available, primarily due to a lack of appropriate nonlinear optical materials.
It is against this background that a need arose to develop the optical devices described herein.